Hydraulic systems frequently employ a hydraulic motor to raise and lower relatively heavy loads and at times to support such loads in an elevated position. When the motor is required to support the load in such elevated position, it is normally desirable to isolate the relatively high load generated pressure in the load supporting end of the motor from the remainder of the system. This is to prevent the downward drifting of the load due to leakage past the valve spools of the conventional control valves normally used in such systems. The load pressure is also normally isolated to prevent the sudden dropping of the load in the event of a line failure or the like.
This isolation is normally accomplished by the disposition of a load check valve in the motor line near or preferably at the load supporting end of the motor. Such load check valve permits free flow of fluid to the motor but normally prevents the escape of fluid therefrom. In some of such systems, a bypass valve is provided adjacent the check valve to bypass fluid therearound for lowering the hydraulic motors.
In earthmoving vehicles such as hydraulic excavators and the like where two or more hydraulic motors are connected to a boom to operate in unison for raising and lowering the load, a load check-bypass valve combination is sometimes mounted to each of the hydraulic motors. It is desirable that the fluid pressure in the motors be substantially equal to prevent uneven operation and distortion of the excavator boom. However, due to manufacturing tolerances, it has heretofore been a problem of assuring that the bypass valves open simultaneously and equally so that the fluid pressure in the motors remain equal during lowering. For example, when the boom is being lowered slowly, one bypass valve may open slightly before the other bypass valve opens, permitting the fluid pressure in the fluid motor controlled by the one bypass valve to decay rapidly with a corresponding increase in the fluid pressure in the other hydraulic motor since it then carries more of the load.